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French economist Thomas Piketty has written an extraordinarily important book. Open-minded readers will surely find themselves unable to ignore the evidence and arguments he has brought to bear.

Capital in the Twenty-First Century contains four remarkable achievements. First, in its scale and sweep it brings us back to the founders of political economy. Piketty himself sees economics “as a subdiscipline of the social sciences, alongside history, sociology, anthropology, and political science”. The result is a work of vast historical scope, grounded in exhaustive fact-based research, and suffused with literary references. It is both normative and political. Piketty rejects theorising ungrounded in data. He also insists that social scientists “must make choices and take stands in regard to specific institutions and policies, whether it be the social state, the tax system, or the public debt”.

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Second, the book is built on a 15-year programme of empirical research conducted in conjunction with other scholars. Its result is a transformation of what we know about the evolution of income and wealth (which he calls capital) over the past three centuries in leading high-income countries. That makes it an enthralling economic, social and political history.

Among the lessons is that there is no general tendency towards greater economic equality. Another is that the relatively high degree of equality seen after the second world war was partly a result of deliberate policy, especially progressive taxation, but even more a result of the destruction of inherited wealth, particularly within Europe, between 1914 and 1945. A further lesson is that we are slowly recreating the “patrimonial capitalism” – the world dominated by inherited wealth – of the late 19th century.

Some argue that rising human capital will reduce the economic significance of other forms of wealth. But, notes Piketty, “ ‘nonhuman capital’ seems almost as indispensable in the twenty-first century as it was in the eighteenth or nineteenth”. Others argue that “class warfare” will give way to “generational warfare”. But inequality within generations remains vastly greater than among them. Yet others suggest that intragenerational mobility robs rising inequality of earnings of significance, particularly in the US. This, too, is false: the rise in inequality of earnings in the US over recent decades is the same however long the period over which earnings are traced. High-school dropouts rarely become chairman of GE.

An important finding is that the ratio of wealth to income in Europe has climbed back above US levels, notably in France and the UK. Another is the notably big recent rise in the income shares of the top 1 per cent in English-speaking countries (above all, the US) since 1980. Perhaps the most extraordinary statistic is that “the richest 1 percent appropriated 60 percent of the increase in US national income between 1977 and 2007.” Technology and globalisation can hardly explain this, since both were at work in all high-income countries. In all, the two most striking conclusions are the rise of the “supermanager” in the US and the return of patrimonial capitalism in Europe.

Third, Piketty uses simple economic models to explain what is going on. He notes, for example, that the huge rise in labour earnings at the top of US income distribution is overwhelmingly explained not by sports stars or entertainers but by increases in remuneration of managers. He argues that this is the result of the falls in marginal taxation, which have increased the incentive to bargain for higher pay, reinforced by changes in social norms. The alternative view – that the marginal productivity of top managers has exploded – is, he asserts, unpersuasive, partly because the marginal product of a manager is unmeasurable and partly because overall economic performance has not improved since the 1960s.

More interesting is Piketty’s theory of capitalist accumulation. He argues that the ratio of capital to income will rise without limit so long as the rate of return is significantly higher than the economy’s rate of growth. This, he holds, has normally been the case. The only exceptions from the past few centuries are when a sizeable part of the return on wealth is expropriated or destroyed, or when an economy has opportunities for exceptionally fast growth, as in postwar Europe or the emerging economies today.

This theory is built on two pieces of evidence. One is that the rate of return is only modestly affected by the ratio of capital to income. In the language of economists, the “elasticity of substitution” between capital and labour is far greater than one. In the long run, this seems plausible. Indeed, an age of robotics might further raise the elasticity.

The other is that, at least in normal times, capitalists save a sufficiently large share of their returns to ensure that their capital will grow at least as fast as the economy. This is especially likely to be true of the seriously wealthy, who are also likely to enjoy the highest returns. Small fortunes are eaten; big ones are not. The tendency for capital to grow faster than the economy is also more likely when the growth of the economy is relatively slow, either because of demographics or because technical progress is weak. Capital-dominated societies also have low-growth economies.

Fourth, Piketty makes bold and obviously “unrealistic” policy recommendations. In particular, he calls for a return to far higher marginal tax rates on top incomes and a progressive global wealth tax. The case for the latter is that the reported incomes of the richest are far smaller than their true economic incomes (the amount they can consume without reducing their wealth). The rich may even take themselves outside any fiscal jurisdiction, so enjoying the fiscal position of aristocrats of pre-revolutionary France. This fact blunts one of the criticisms of the book’s reliance on pre-tax data: over time, the ability of individual countries to redistribute resources towards the middle and bottom of national income distributions might dwindle away to nothing.

Yet the book also has clear weaknesses. The most important is that it does not deal with why soaring inequality – while more than adequately demonstrated – matters. Essentially, Piketty simply assumes that it does.

One argument for inequality is that it is a spur to (or product of) innovation. The contrary evidence is clear: contemporary inequality and, above all, inherited wealth are unnecessary for this purpose. Another argument is that the product of just processes must be just. Yet even if the processes driving inequality were themselves just (which is doubtful), this is not the only principle of distributive justice. Another – to me more plausible – argument against Piketty’s is that inequality is less important in an economy that is now 20 times as productive as those of two centuries ago: even the poor enjoy goods and services unavailable to the richest a few decades ago.

For me the most convincing argument against the ongoing rise in economic inequality is that it is incompatible with true equality as citizens. If, as the ancient Athenians believed, participation in public life is a fundamental aspect of human self-realisation, huge inequalities cannot but destroy it. In a society dominated by wealth, money will buy power. Inequality cannot be eliminated. It is inevitable and to a degree even desirable. But, as the Greeks argued, there needs to be moderation in all things. We are not seeing moderate rises in inequality. We should take notice.